Jeremy Bamber handcuffed to a police escort in 1988. Photograph: Today/Rex Features

Some of the world's most eminent ballistics experts have uncovered "the first evidence directly pointing to the innocence of Jeremy Bamber", convicted of a notorious multiple murder 27 years ago.

Bamber was found guilty in October 1986 of shooting his adoptive parents, June and Nevill, his sister Sheila Caffell and her six-year-old twins, Daniel and Nicholas, at their Essex farmhouse. He has consistently maintained his innocence, although his last attempt to win freedom was rejected by the Criminal Cases Review Commission 12 months ago. However, a new legal team has submitted evidence to the commission, the authority that investigates miscarriages of justice, claiming to have unearthed evidence that "shakes the safety of Bamber's convictions to their core".

Detailed reports, compiled by British and US medical and ballistics analysts, corroborate the initial police view that Bamber's schizophrenic sister Sheila Caffell committed the White House Farm murders in 1985. During the immediate aftermath of the killings on 7 August, detectives and the pathologist thought Caffell, 28, had murdered her parents and sons before turning the gun on herself.

Yet the theory was cast into doubt when three days after the shootings a cousin of Bamber found a silencer in a cupboard at the farm, apparently with Caffell's blood on it. Central to the prosecution case in the Chelmsford crown court trial was evidence that Caffell's blood was on the silencer; if so, she could not have shot herself then placed it in a cupboard downstairs. Jurors heard how the silencer was responsible for scratch marks on a kitchen shelf, allegedly made in a struggle between Bamber and his 61-year-old adoptive father.

The trial was unusual in that the jury were told the killings could have been carried out only by Caffell or Bamber. The issue of the silencer was vital in persuading the jury, with the judge instructing them the silencer "could, on its own, lead them to believe that Bamber was guilty".

Now the conclusions of a peer-reviewed pathology assessment of the evidence relating to Caffell's death, obtained by the Observer, appear to demolish the case against Bamber. They suggest that a silencer – so pivotal to Bamber's conviction – was never used. One report by David Fowler, chief medical examiner of the US state of Maryland, who has reviewed the files of more than 3,000 shooting homicides, states: "In my professional opinion, the [burn marks] complex just described of the lower entrance and two abrasions is consistent with the rifle not having a silencer."

Fowler believes no silencer was involved. His conclusion is supported by Ljubisa Dragovic, chief medical examiner of Oakland county in Michigan, and Marcella Fierro, former chief medical examiner of Virginia.

Leeds-based Simon McKay, Bamber's new solicitor advocate, said: "The evidence of three senior and respected pathologists that the wounds to Sheila Caffell are consistent with the rifle having been fired without the silencer fitted shakes the safety of Jeremy Bamber's convictions to their core."

McKay added: "The fresh expert evidence aligns itself with what police officers found at the scene on the morning of the killings and the combined views of those who assessed the position then: namely, and tragically, [that] Sheila Caffell murdered her family, then took her own life."

Evidence that the fatal wounds had been fired by a rifle without a silencer are corroborated by further fresh analysis of burn marks on Nevill Bamber's back. The findings are supported by firearms experts working for Dr John Manlove, an Oxfordshire-based forensic scientist.

Manlove states: "From its size and shape, this mark could possibly have been caused by the hot muzzle of a firearm, without a sound moderator." He says that further testing is required with the murder weapon, an Anschütz 525 rifle, to underpin his initial assessment.

Manlove's conclusions are corroborated by gunfire tests conducted last month in Arizona. A report by Daniel Caruso, chief of burn services at the Arizona Burn Centre and executive chair of the department of surgery at the University of Arizona, states: "In my professional opinion, the three wounds sustained by Ralph [Nevill] Bamber are consistent in size, shape and diameter with a threaded end of a model 525 Anschütz rifle barrel heated sufficiently to cause injury."

McKay is adamant that the CCRC has no option but to refer the case.

During the trial, the jury struggled to reach a verdict, requesting to see the evidence relating to the blood on the silencer, before returning with a 10-2 majority. McKay added: "A picture is emerging that exculpates Jeremy Bamber and implicates his sister." Until the finding of the silencer, he says, there was no reason to doubt the initial view of detectives that Caffell committed the murder then killed herself. The pathologist, Dr Peter Vanezis, added: "My examination did not reveal anything to contradict the suicide theory."

Although the burn marks were raised at the trial, McKay said they were dismissed as a "mystery". A senior forensics expert in 1985 raised the possibility that the rifle muzzle may have been responsible but no evidence exists that he pursued this line of inquiry. Bamber's lawyers have recently obtained a copy of a note from the Home Office database endorsing that tests were needed to ascertain how hot the silencer became after firing, but again no proof is available that this was pursued.